Superconducting magnet coil



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' 1 SUPERCONDUCTI'NG MAGNET COIL Filed Jan. 28, 1965 NORMAL METALCOATING United States Patent Filed Jan. 28, 1965, Ser. No. 428,676Claims priority, applicatiognzgermany, Jan. 30, 1964,

7 Claims. (ci. 335-216) Our invention relates to superconducting magnetcoils.

With such coils, particularly those of large dimensions, aninadvertently or intentionally effected transition from superconductanceto normal conductance may result in high voltages and heatconcentrations at individual parts or localities of the coil winding.These occurrences tend to damage or destroy the coil insulation or tomelt the superconductor at individual points, thus rendering the coilinoperative.

It is an object of our invention to device a superconducting magnet coilin which excessive voltages and excessive heat concentrations inindividual parts of the winding, due to occurrence of transition, areeffectively prevented.

According to the invention, we provide a plurality of ohmic resistancebridges between adjacent windings or turns of the superconducting coilso as to secure a rapid propagation of the transition; the resistance ofthese bridges being sufiiciently large to prevent the current flowingthrough the bridges during the build-up period of the coil excitationand then heating the coil, from reaching a value detrimental to thesuperconducting property of the coil. On the other hand, the absoluteresistance values of the ohmic bridges are kept lower than theresistance of an individual portion of coil wire as may initiallyconvert to normal conductance, such a wire portion being short incomparison with the length of a winding turn.

That is the material of the resistance bridges has a higher specificresistance than the coil material when the latter is in the condition ofnormal conductance, but the resistance bridges nevertheless have asmaller absolute resistance value than a given short length of such acoil-wire in normal conductance, for example a wire piece of 1centimeter length. The resistance bridges which thus may form a currentpath from winding to winding or turn to turn in the vicinity of such ashort coil-wire piece in transition to normal conductance, arepreferably about 5 to 50 micron long and are interposed between theindividual Winding turns.

The ohmic resistance bridges according to the invention may be locatedat several individual points of each winding turn or they may extendover the entire periphery of the turn. It will be understood that theresistance of the bridges must be at least so large that, when thecurrent intensity gradually increases while the excitation of the coilis being built up, the current flowing through the bridges produceswithin the coil only a small amount of heat which can readily bedissipated by the surrounding cryogenic medium, such as a bath of liquidhelium, and hence without appreciably increasing the temperature of thesuperconducting coil material. For this reason, massive resistancebridges of silver, copper and the like good conducting materials are notsuitable for the resistance bridges. These bridges rather consist ofmaterial having a much higher specific resistance such as a coating ormixture containing carbon granules, as will be more fully describedhereinafter.

The superconducting material of the magnet coil may consist of any metalor alloy known for such purposes. For example, when forming thesuperconducting coil of niobium-zirconium or niobium-tin alloy, atemperature increase from 4.2 to 5.2 K. is still permissible because itdecreases the loadability of the winding only by a slight percentage,namely by less than 15% in the example just mentioned. It is thereforeof advantage in some cases to reduce the rate of current increase,during starting-up of the excitation of a superconducting magnet coilequipped with resistance bridges according to the invention, down to afraction of the initial rate of current increase, as soon as the currentapproaches the critical intensity value at which the transition is aboutto occur.

As explained, the absolute resistance of the ohmic bridges according tothe invention is considerably smaller than that of a short coil-wirepiece, for example in the dimensional order of 1 centimeter, whichduring transition has converted to normal conductance. If under theseconditions the transition occurs in a piece of a few centimeters, forexample, the absolute resistance of this wire piece increases to amultiple of the resistance value of the resistance bridges between thewindings. Thus, the shunt path now formed by the ohmic resistancebridges and the still superconducting adjacent winding turns, withrespect to the coil piece already converted to normal conductance, has alower resistance than the now normalconducting coil piece. The conductorcurrent therefore commutates onto the adjacent winding turns with a timeconstant determined by the inductivity and the resistance difference ofthe two parallel current paths. With the usual wire dimensions, thistime constant is in the order of magnitude of l0 second. As soon as anadjacent winding turn is loaded in this manner with commutated currentin addition to the current already traversing this winding, thecondition for transition in the neighboring winding turn is establishedor promoted. As a result, the phenomenon of transition is rapidlypropagated through the entire coil layer within a few microseconds.

In this manner, the magnetically active current flow or flux in thewindings of one layer is substituted by a transverse current passingthrough the ohmic resistance bridges. Since this transverse currentflows parallel to the coil axis, it does not by itself produce amagnetically active fiow (flux). For that reason, the current in theother layers of coil windings increases. Consequently, at thoselocalities where the current is closest to the critical current value,new transitions are released. These again propagate within a fewmicroseconds throughout the particular coil layer. Thus an entiremulti-layer coil converts to normal condition within fractions ofmilliseconds. When this condition is reached, the resistance bridges arerelieved of the transverse current because the voltage induced in eachwinding is now consumed by its own ohmic resistance. Due to the shortduration of the total transition period, the heat concentration in thelocality first convered to normal conductance is still harmless.

As to the magnitude of the resistance value to be given to the ohmicbridges according to the invention, the following condition issignificant: The resistance value is preferably made so small that theproduct of the number of winding turns, the resistance value of thebridge between two winding turns, and the rated current of the coil, issmaller than the breakdown voltage of the winding insulation at the mostunfavorable spot. Depending upon the rate of current increase, the sizeof the coil, the number of windings, the insulation and the coolingconditions, the resistance value of the bridges is approximately between0.1 milliohm and 1 ohm. Only under extreme con ditions is it advisableto select a resistance value outside of this range.

As explained, the resistance between two mutually adjacent windings isformed by bridges of material which has a relatively poor conductivity,This material consists for example, of a conducting varnish with whichthe superconducting wire of the coil is coated, and/or of acarbon-containing paste interposed between winding turns wound in spacedrelation to each other. The layer of conducting varnish with which thewire is coated may have a thickness of 10 micron, for example.Preferably, the materials for the resistance bridges consist of finegranular graphite or other carbon or also of granular metal, or mixturesof such substances distributed. colloidally in insulating varnish orsynthetic resinous plastic of the kind conventionally employed inelectrical equipment for insulating purposes. Such resistance bridges indry condition and at the cryogenic temperatures of the superconductor,for example at 4.2 K., exhibit specific resistances of 0.1 to 10 ohm-cm,depending upon the dimensions of the coil and the wire.

Another way of providing for the ohmic resistance bridges is tosimultaneously wind a thread of glass fiber or glass wool, particularlya glass-fiber material impregnated with conducting varnish or conductingcasting resin or potting resin, together with the superconducting wireso that the glass-fiber thread is firmly located between adjacentwinding turns on the spool or carrier of the winding. The glass-fiberthread is preferably made slightly thicker than the wire. When thelayers of coil windings are being wound upon each other, the thread isfirmly pressed against the wire thus providing for a sufiicient contactengagement between wire and thread. If it is desired to produce theohmic resistance bridges not uniformly along the entire periphery of thewinding turns, but only at individual localities, the impregnation ofthe thread is to be effected only at these particular localities.

The invention will be further explained with reference to embodiments ofsuperconducting magnet coils according to the invention illustrated byway of example on the accompanying drawing in which:

FIG. la shows schematically and in section a portion of a first coil,and FIG. 1b is a corresponding cross-sectional view.

FIGS. 2, 3 and 4 show respectively. three other embodiments also bypartial views and in section.

In all embodiments, the spool or carrier of the coil is denoted by 1,insulating intermediate layers between the individual winding layers aredenoted by 2, the coil axis by 3, the cross section of the baresuperconducting coil wire by 4. The coating of the superconducting coilwire is denoted by 5, this coating having only poor conductance incomparison with the wire itself. Disposed between the individual layersof windings are copper foils 6.

In the embodiment according to FIGS. la and lb, the layers of the coilwinding are individually insulated within the winding space of thespool 1. The spool structure may consist of copper, for example, and belined with an insulating foil 2a. Placed between the winding layers areinsulating intermediate layers 2. These consist of a material of goodmechanical stability, name-1y with respect to pressure, tearing andbending forces, at all occurring operating temperatures including thosein the vicinity of absolute zero. Particularly well suitable as materialfor the intermediate layer 2 is polyethylene terephthalate. Indistinction from conventional magnet coils, the superconducting wires ofniobium-zirconium alloy are not insulated but are provided with acoating 5 of relatively poor conductance, which consists of theabovementioned conductive varnish or similar material, to secure thedesired transfer resistance from turn to turn.

It is further preferable to have the superconducting wire piece whichinterconnects two adjacent winding layers of the coil, contacted by ametallic conductor, particularly a copper foil. Such a foil is shownwound onto the wire piece at 11 in FIG. lb. The metallic conductor thencommences to conduct the current from the wire piece which interconnectsthe two layers, as soon as these layers have converted to the conditionof normal conductance. This is advantageous because the mentionedtransfer localities between two winding layers are not tion andtherefore are particularly jeopardized in the event of transitionoccurring at these localities.

In the embodiment of FIG. 2, each winding layer is separately insulatedwith respect to the spool structure 1 and relative to the copper foils 6placed between the winding layers. The copper foils may beshort-circuited by being electrically connected with each other, or theymay also be left unconnected. The heat generated in the coil isconducted by the copper foils to the spool structure 1 and thence to asurrounding bath of liquid helium (not illustrated). The thickness ofthe copper foils is chosen in dependence upon the axial length of thecoil and is about 5 to 50% of the coil-wire diameter, the thicker foilsbeing used with the longer coils.

To prevent the copper foils from being electrically in conductingconnection with the spool structure, an insulating lining 2b ispreferably provided between the spool structure and the foils, the edgesof the foils being bent upwardly as illustrated. For the same reason,namely for preventing short-circuit currents, the copper foils may beslitted along their edge in parallel relation to the coil axis up to themiddle of the coil, such slits being shown at 12 in FIG. 2.

The use of thermally and electrically good conducting foils, such as theabove-described copper foils, is also of advantage without the conjointprovision of the abovementioned ohmic resistance bridges. That is, suchfoils are also applicable to advantage in other superconducting coilsfor the dissipation of heat and/ or the rapid propagation of atransition once it has occurred. In a coil whose conductors are coatedwith a good heat-conducting material, the foils are preferably employedfor heat dissipation by placing them electrically insulated between thelayers of coil windings. Furthermore, such coils, if placed in contactdirectly with the coil wires which are coated with conducting varnish orsimilar material, may contribute to rapid propagation of a transition aswill be further explained below with reference to FIG. 3.

In the embodiment of FIG. 4, the resistance bridges between the windingturns 4 of bare superconducting wire, are formed by a paste 7. Afterwinding of each individual layer of turns, the paste is inserted intothe spaces between the turns which are kept suitably spaced from eachother. The resistance paste may contain colloidally distributed graphiteor other carbon material and/ or metal as described above.

In the embodiment of FIG. 3, several axially short component coils 9 areplaced about a cylindrical spool or coil carrier 8. Each component coil9 has only a relatively small number of winding turns 4 per layer, forexample a few hundred turns, the wire being coated with conductingvarnish 5. The resistance bridges are formed not only between each turnand the adjacent turn, but are effective through intermediate layers 6of copper between each turn and all other turns of the same layer.Although this results in a larger amount of parallel current when thecoil is being charged up, the heat dissipation is particularly welleffective by virtue of the axially short intermediate layers of copperwhich directly contact a bath of liquid helium (not illustrated). Inthis manner, the large heat losses occurring during the build-up periodof the coil excitation can be dissipated without appreciable increase intemperature. The free space-between the intermediate layer 6 of copperand the wires 4 coated with varnish 5 may be filled with a resistancepaste as mentioned above with reference to FIG. 2.

In the embodiment of FIG. 4, the resistance bridges are formed with theaid of threads 10 which are wound between the wire turns 4 and have arelatively poor conductance. The threads consist of glass wool orsimilar glass-fiber material impregnated with the above-mentionedconducting varnish or conducting synthetic resinous plastic. If desired,the glass-fiber threads may be impregnated in this manner only locallyalong a length of about one centimeter, each impregnated locality beingspaced about ten centimeter from the next impregnated locality. Theglass-fiber threads have a somewhat larger diameter than the wires 4and, during winding operation, are firmly pressed between the wire turnsand the insulating intermediate 'layers 2, so that a sufiicientelectrical contact between the threads and the superconducting wire issecured. The remaining free space between the insulating layers 2, thewires 4 and the threads 5, may be filled with a resistance paste asdescribed above with reference to FIG. 2.

It may happen that the surface of the bare superconducting wire sufferscorrosion, for example by oxidation or the elfect of nitrogen. As aresult, the surface resistance of the wire may have an order ofmagnitude equal to, or larger than, that of the resistance bridgesbetween the individual winding turns. Under conditions where suchcorrosion is to be expected, it is advisable to provide thesuperconducting wire with a protective coating, preferably of silver,gold or copper. The thickness of such coating should be approximatley0.5 to 5% of the wire cross section. The coating may be deposited duringdrawing of the wire or by electrolytic deposition. Aside from theimprovement with respect to the transverse resistance between theindividual windings thus obtained, the metallic coating on thesuper-conducting wires also improves the heat condutance and thuspromotes a more rapid propagation of the transition along thesuperconducting.

As mentioned, the resistance bridges provided according to the inventionbetween the windings or turns of a superconducting magnet coil, not onlyreduce voltage stresses during transition, but also prevent excessiveheat concentration at individual localities of the coil. The magneticenergy of a large coil, if uniformly distributed over the entire coil,may increase the temperature of the superconductor by several hundreddegrees Kelvin. The temperature increase, however, can be limited to amaximum of about 40 K. by increasing the coil mass, for example byinterposed copper inserts or by providing heat-conductivelyinter-communication hollow spaces filled with evaporating liquids suchas nitrogen or water. If no local overheating occurs, a limit of about400 K. is not detrimental to the insulation of the windings. Withoutspecial expedients, however, the transition, commencing at one locality,propagates only over a small fraction of the entire wire length, and thetemperature increase in this considerably much smaller volume is muchhigher than may have been computed for the entire coil.

Consequently, without the benefit of the present invention, not only theinsulation of the superconducting coil is endangered, but the wireitself may melt locally despite the fact that this require temperaturesof 2000" K.

or more.

The resistancebridges provided according to the invention betweenadjacent windings take care of having the transition rapidly propagatedover large portions of the coil, particularly rapidly throughout eachindividual layer of winding turns. This requires that a bridge to theadjacent winding is present not only at a single locality of thewinding, but at least at several localities of the periphery. It is mostfavorable if the ohmic resistance bridges between the windings virtuallyextend over the entire periphery. For very large windings, for examplewith more than 1 meter wire length per turn, it is advisable, however,to provide the resistance bridges only at individual localities becauseotherwise the total resistance of the ohmic bridge, if continuouslyextending between the windings over the entire periphery, would be toosmall.

To those skilled in the art it will be obvious upon a study of thisdisclosure that our invention permits of various modifications and maybe given embodiments other than particularly illustrated and describedherein, within departing from the essential features of our inventionand within the scope of the claims annexed hereto.

We claim:

1. A superconducting magnet coil comprising layers of windings ofsuperconducting material mutually insulated from each other and havingaxially sequential winding turns, and a plurality of ohmic resistancebridges interposed between adjacent turns and resistivelyinterconnecting said turns, said bridges having a high resistancerelative to that of a respective winding when said winding is insuperconducting condition so as to limit the bridge current to apermissible value during build-up periods of coil excitation, and saidbridge resistance being lower than that of a given fractional length ofa single turn when in condition of normal conductance, whereby excessivevoltage and heat concentration due to local initiation of transition inthe winding is prevented, said resistance bridges being formed ofcomposition material containing embedded fine-granular substance fromthe group consisting of carbon and metal.

2. A superconducting magnet coil comprising layers of windings ofsuperconducting material mutually insulated from each other and havingaxially sequential winding turns, and a plurality of ohmic resistancebridges interposed between adjacent turns and resistivelyinterconnecting said turns, said bridges having a high resistancerelative to that of a respective winding when said winding is insuperconducting condition so as to limit the bridge current to apermissible value during build-up periods of coil excitation, and saidbridge resistance being lower than that of a given fractional length ofa single turn when in condition of normal conductance, whereby excessivevoltage and heat concentration due to local initiation of transition inthe winding is prevented, said resistance bridges comprising a body ofsynthetic plastic with embedded and colloidally dispersed fine-granularsubstance from the group consisting of carbon and metal.

3. A superconducting magnet coil comprising layers of windings ofsuperconducting wire mutually insulated from each other and havingaxially sequential and mutua-lly spaced winding turns, and a pluralityof ohmic resistance bridges interposed between adjacent turns andresistively interconnecting said turns, said bridges comprising a bodyof synthetic plastic with embedded and dispersed fine-granular substancefrom the group consisting of carbon and metal, the resistance of saidbridges being higher than that of a respective winding when said windingis in superconducting condition so as to limit the bridge current to apermssible value during build-up periods of coil excitation, and saidbridge resistance being lower than that of a given fractional length ofa single turn when in condition of normal conductance, whereby excessivevoltage and heat concentration due to local initiation of transition inthe winding is prevented.

4. A superconducting magnet coil comprising a winding of superconductingmaterial having axially sequential winding turns, and ohmic resistancebridges interposed between adjacent turns and resistivelyinterconnecting said turns, said bridges having a high resistancerelative to that of said winding when said winding is in superconductingcondition so as to limit the bridge current to a permissible valueduring build-up periods of coil excitation, and said bridge resistancebeing lower than that of a, given fractional length of a single turnwhen in condition of normal conductance, whereby excessive voltage andheat concentration due to local initiation of transition in the windingis prevented, said resistance bridges comprising glass-fiber threadswound between said turns and in contact therewith, said threadscontaining a resistively conducting impregnation to provide for saidbridge resistance.

5. In a superconducting magnet coil according to claim 4, saidimpregnation being located at mutually spaced localities of said thread.

6. A superconducting magnet coil comprising layers of windings ofsuperconducting material mutually insulated from each other and havingaxially sequential winding turns, and a plurality of ohmic resistancebridges interposed between adjacent turns and resistivelyinterconnecting said turns, said bridges having a high resistancerelative to that of a respective winding when said winding is insuperconducting condition so as to limit the bridge current to apermissible value during build-up periods of coil excitation, and saidbridge resistance being lower than that of a given fractional length ofa single turn when in condition of riorrnal conductance, wherebyexcessive voltage and heat concentration due to local initiation oftransition in the winding is prevented, said resistance bridges beingformed of a layer of material consisting of conducting varnish.

7. A superconducting magnet coil comprising layers of windings ofsuperconducting material mutually insulated from each other and havingaxially sequential winding turns, and a plurality of ohmic resistancebridges interposed between adjacent turns and resistivelyinterconnecting said turns, said bridges having a high resistancerelative to that of a respective winding when said winding is insuperconducting condition so as to limit the bridge current to apermissible value during build-up periods of coil excitation, and saidbridge resistance being lower than that of a given fractional length ofa single turn when in condition of normal conductance, whereby excessivevoltage and heat concentration due to local initiation of transition inthe winding is prevented, said resistance bridges being formed of alayer of material consisting of insulating varnish containing embeddmifine-granular substance selected from the group of substances consistingof carbon and metal.

References Cited UNITED STATES PATENTS 3,187,235 6/1965 Berlincourt eta1. 3352l6 3,255,335 6/1966 Kortelink 3352l6 X 3,263,133 7/1966 Stekly335-2l6 X

1. A SUPERCONDUCTING MAGNET COIL COMPRISING LAYERS OF WINDINGS OFSUPERCONDUCTING MATERIAL MUTUALLY INSULATED FROM EACH OTHER AND HAVINGAXIALLY SEQUENTIAL WINDING TURNS, AND A PLURALITY OF OHMIC RESISTANCEBRIDGES INTERPOSED BETWEEN ADJACENT TURNS AND RESISTIVELYINTERCONNECTING SAID TURNS, SAID BRIDGES HAVING A HIGH RESISTANCERELATIVE TO THAT OF A RESPECTIVE WINGING WHEN SAID WINDING IS INSUPERCONDUCTING CONDITION SO AS TO LIMIT THE BRIDGE CURRENT TO APERMISSIBLE VALUE DURING BUILD-UP PERIODS OF COIL EXCITATION, AND SAIDBRIDGE RESISTANCE BEING LOWER THAN THAT OF A GIVEN FRACTIONAL LENGTH OFA SINGLE TURN WHEN IN CONDITION OF NORMAL CONDUCTANCE, WHEREBY EXCESSIVEVOLTAGE AND HEAT CONCENTRATION DUE TO LOCAL INITIATION OF TRANSITION INTHE WINDING IS PREVENTED, SAID RESISTANCE BRIDGES BEING FORMED OFCOMPOSITION MATERIAL CONTAINING EMBEDDED FINE-GRANULAR SUBSTANCE FROMTHE GROUP CONSISTING OF CARBON AND METAL.